1. Field of the Invention
This invention relates to etching features into a semiconductor substrate, and in particular to a method and workpiece for precisely etching an aperture of known cross-sectional area through the substrate.
2. Description of the Prior Art
Injector nozzles for ink jet printers and fuel injectors for internal combustion engines require precise metering of injected fluids, which in turn requires a precise aperture of known cross-sectional area and shape. While silicon micromachining technology allows for relatively precise control of the etching process for the aperture, improved accuracy is often not realized because of the misalignment of the pre-etch mask opening with the crystal lattice. While a circular mask would alleviate the requirement to align to the crystal lattice, computer generated masks can not be generated with a perfect circle. Any polygon used as an approximation for the mask aperture will have straight sides that are misaligned from the crystal lattice because of the limited accuracy of tools used for aligning the mask and the substrate material.
The crystal orientation of the silicon wafer causes a rectangular aperture to be generated by the etching process regardless of the polygon used in the pre-etch mask pattern. If the original mask aperture is chosen as a rectangle or square, then the sides of the polygon must align exactly with the &lt;111&gt; planes of the wafer. Any misalignment results in the final aperture being a square having the largest dimensions in each &lt;111&gt; direction of the original, misaligned square aperture in the pre-etch mask. See FIG. 3 for a visual representation of the rotation of the pre-etch mask aperture and the process by which it creates an aperture of enlarged cross-sectional area in the silicon substrate.
Poteat, in U.S. Pat. No. 4,470,875, discloses a method for constructing an alignment indicator in the silicon wafer to aid in the process of aligning the aperture in the pre-etch mask more precisely with the crystal lattice within the substrate. However, the Poteat process does nothing to improve the mechanical misalignments inherent in the equipment used for aligning the pre-etch mask with the silicon wafer, even when the precise crystal orientation is known. This misalignment is especially significant when it is necessary to etch cooperating apertures through the substrate from opposite sides of the wafer. It is therefore an object of the present invention to define a process for eliminating the enlargement introduced in an aperture etched in a substrate when the aperture in the pre-etch mask is misaligned with respect to the crystal lattice in the substrate.